In recent years, regulations relating to gas emissions from the engines of hydraulic work machines such as hydraulic excavators have been more tightened with each passing year. Market needs for improved fuel economy are also becoming stronger with the rising fuel costs and the continuing business recession.
In response to the gas emissions regulations, engine manufacturers and others have made efforts toward the reduction of the particulate matter (PM) and nitrogen oxides (NOx) contained in gas emissions, in particular, and techniques for sophisticating combustion control have been developed in large numbers to date. At the same time, technology has also been developed that is intended to collect and purify the above particulate matter (PM) and nitrogen oxides (NOx) by placing a gas emissions aftertreatment device(s) such as a diesel particulate filter (DPF) and a urea SCR (Selective Catalytic Reduction) system, between the engine and muffler of the vehicle. This technology is combined with a combustion control sophistication technique, as appropriate, to meet the increasingly tightened gas emissions regulations.
The gas emissions aftertreatment devices such as DPFs and urea SCR systems, however, usually use complex and expensive materials. For example, the catalysts in DPFs use platinum. Additionally, SCR systems need to have a tank for storage of urea, and a urea injector. Accordingly, engine systems having a gas emissions aftertreatment device(s) are considerably expensive, compared with engine systems not having one. Establishment of a method for reducing PM, NOx, and other regulated gas components from the engine exhaust itself, therefore, is being desired partly for the deletion or simplification of aftertreatment devices.
For improved fuel efficiency, on the other hand, hybrid-driven hydraulic work machines having an electric assist motor driven by a battery or any other elect storage device besides an engine, as driving sources, are being proposed or developed. In the hybrid of this configuration, the output power required for a pump is supplied from the engine not only independently, but also with the assist motor. Traditionally, engine torque has been determined according to a particular output of a hydraulic pump. Use of an electric assist motor, however, allows a desired engine torque value to be set, which is to say, an operating point of the engine that is represented by the engine speed and torque can be set at a fuel-efficiently favorable position.
For example, the construction machine proposed as a hydraulic work machine in Patent Document 1 includes an electric motor driven by an engine and is designed to save energy by storing excess engine output power as electrical energy, and then when the engine output power is insufficient, releasing the stored electrical energy and driving the electric motor to maintain the necessary torque absorbed by a pump. According to Patent Document 1, with this machine configuration, a compact engine that generates rated output power equivalent to average horsepower needed for the construction machine to perform work can be adopted for improved fuel economy and reduced CO2 emissions.
The working machine proposed in FIG. 6 of Patent Document 2 is constructed to generate an oil pressure by driving a hydraulic pump via an engine and an electric motor. This working machine employs a control method that includes setting an Increase rate of the engine output power as a predetermined value, then comparing a maximum achievable value of the engine output power that is calculated from the predetermined value of the increase rate, with an output power requirement calculated from the hydraulic output power required for the hydraulic pump, and if the calculated output power requirement exceeds the maximum achievable value of the engine output power, compensating for this excess output power with output power of the motor. According to Patent Document 2, with this machine configuration, even if a hydraulic load abruptly increases, a decrease in combustion efficiency, occurrence of particulate matter, and a stoppage of the engine can be avoided since operating conditions of the engine can be maintained in an adequate range by controlling the engine load so as to avoid its abrupt increase.